软土地区大直径超长后注浆钻孔灌注桩竖向承载力的试验研究

桩端后注浆是软土地区大直径超长钻孔灌注桩发展的必然趋势。依据软土地区某超高层建筑大直径超长钻孔灌注桩试桩的竖向抗压静载试验结果,通过对基桩的竖向极限承载力性状、桩身轴力传递特性以及桩侧阻力、桩端阻力发挥特性的分析研究,揭示了桩端后注浆对基桩承载性状的影响情况,深入探讨了软土地区大直径超长后注浆灌注桩的荷载传递机理和竖向承载性状,并提出了单桩承载力的估算方法。     

上海地区超长后注浆钻孔桩竖向承载力试验研究

桩端注浆是上海地区大直径超长灌注桩发展的必然趋势。依据上海地区某超高层商业建筑试桩的高吨位竖向抗压静载试验结果,通过对基桩的竖向极限承载力性状、桩身轴力传递特性以及桩侧阻力、桩端阻力发挥特性的分析研究,揭示了桩底注浆对基桩承载性状的影响情况,深入探讨了上海软土地区大直径超长后注浆灌注桩的荷载传递机理和竖向承载性状,并提出了单桩承载力的估算方法。     

后注浆超长大直径钻孔灌注桩承载性能试验研究

以浙江沿海软土地区某高层中桩基础为工程依据,进行了两根后注浆超长大直径灌注桩的堆载静载试验和一根桩的自平衡静载试验。通过对试验数据的采集整理,探讨了软土地基超长大直径钻孔灌注桩注浆后的竖向承载特性。结果表明,桩端注浆使软土地区超长大直径桩由典型的摩擦桩变为端承摩擦桩,从而改善了超长桩竖向承载特性,提高了该类桩承载性能的稳定性。     

昆明景成大厦大直径超长灌注桩设计及承载特性研究

介绍昆明景成大厦的大直径超长灌注桩设计、轴力测试及单桩竖向抗压静载试验等情况,根据测试结果对单桩承载特性进行分析,研究大直径超长灌注桩极限侧阻力、端阻力分布及不同荷载水平下侧阻力、端阻力的发挥情况,以及后注浆工艺对单桩承载力的影响,并对试桩采用双套筒方式消除无效桩段的效果进行了分析。研究表明,由于超长桩的桩身压缩量较大使下部桩变形量偏小,导致下部桩侧阻力和端阻力不能充分发挥;采用后注浆工艺可显著提高超长灌注桩承载力,增大注浆量可提高超长灌注桩承载力并可改变桩侧阻力和端阻力的分布,采用在桩端部和下段进行后注浆可提高超长灌注桩端部承载力的发挥效率,并提出超长桩的静载试验不宜以总沉降量作为终止加载条件。     

桩端后注浆工艺对钻孔灌注桩承载性状的影响

通过静载荷试验和桩身轴力测试,对比了注浆及未注浆两种情况下灌注桩承载性状,说明桩端注浆后大大提高了软土地区钻孔灌注桩的单桩承载力。最后分析了注浆后桩侧摩阻力、桩端阻力提高的原因。     

后注浆超长钻孔灌注桩抗压承载力试验研究

以天津地区某在建超高层的桩基静载试验为基础,对软土地区后注浆超长钻孔灌注桩的承载性能进行了研究。结果表明:软土地区的超长钻孔灌注桩为纯摩擦桩;超长钻孔灌注桩桩侧摩阻力和桩端阻力的发挥是不同步的,桩侧摩阻力的发挥先于桩端阻力;超长钻孔灌注桩的桩顶沉降量很大,应采用桩顶沉降量来确定其极限承载力;超长钻孔灌注桩存在有效桩长,其长径比不超过80是比较合理的。     

上海中心大厦大直径超长灌注桩现场试验研究

中国在建第一高楼上海中心大厦（ 632 m ）采用了直径为 1 m 、桩端埋深 88 m 的大直径超长灌注桩,有别于金茂大厦（ 420 m ）、上海环球金融中心（ 492 m ）另两栋超高层建筑所采用的钢管桩。通过现场试桩验证成桩可行性及承载力取值,试桩载荷试验加载至极限,采用分布式光纤量测桩身应变,同时为研究上海软土地区大直径超长灌注桩承载特性及荷载传递机理提供了有价值的数据。试验结果表明：试桩破坏前,Q – s 曲线近似为线性,破坏时,桩体发生刺入变形; 桩侧桩端联合后注浆桩与桩端后注浆桩在侧摩阻力分布及发挥性状方面存在显著差异; 黏性土中桩侧摩阻力充分发挥所需桩土相对位移小于 5 mm ,砂性土中小于 10 mm ;桩土相对位移超过极限位移后,埋深较浅的黏性土中由于桩土相对位移大出现明显的软化现象;与规范值相比,有效桩长范围内浅部土层中桩侧摩阻力小于规范取值下限,深部土层中桩侧摩阻力达规范取值上限的 2 倍以上;试桩端阻比较小,表现出摩擦型桩特性;桩身压缩占桩顶沉降 95% 左右,桩顶沉降主要由桩身压缩产生。试桩试验为上海软土地区 600 m 超高层建筑首次采用灌注桩提供指导和技术支持。     

基于桩侧广义剪切模型的大直径超长灌注桩承载变形计算方法

大直径超长灌注桩桩身变形较大,桩侧与土体易出现明显的界面滑移,传统剪切位移法难以适合其承载变形计算。基于大直径超长灌注桩桩–土剪切作用性状及桩侧摩阻力发挥特点,采用剪切位移和剪切滑移两阶段法描述其桩侧摩阻力发挥过程,形成桩侧广义剪切模型;在此基础上,采用传递矩阵增量方式建立大直径超长灌注桩承载变形计算方法,并给出计算参数的取值。该方法考虑了桩侧摩阻力发挥的非线性、桩端承载的非线性及桩身材料的非线性,并考虑了桩–土滑移后桩侧摩阻力软化特性及桩端后注浆对桩端承载性状的影响。工程实例计算结果与现场试桩实测值较为吻合,表明基于桩侧广义剪切模型建立的大直径超长灌注桩承载变形计算方法具有合理性与可行性。     

桩端后注浆大直径超长钻孔灌注桩现场试验研究

通过对3根端后注浆大直径超长灌注桩的现场试验,采用双循环法进行加载,采用沉降杆法测试工程桩顶、桩底的沉降,同时通过应力计测试桩身不同断面的轴力。为研究软土地区大直径超长灌注桩提供有价值的数据。试验表明:桩底沉降较小,最大加载时约为3mm,试桩顶的累计沉降几乎全部由桩身压缩量产生;桩端后注浆处理后,由于桩底沉降较小,桩身桩土相对位移减小,使桩侧土层摩阻力相对于不考虑后注浆时有明显提高,提高比例约为40%~50%,桩端阻力约占总承载力的24%。     

广东地区大直径超长钻孔灌注桩荷载传递特性试验研究

 通过广东软土地区大直径超长钻孔灌注桩大吨位静载试验,分析了该地区大直径超长钻孔灌注桩承载特性及荷载传递机制,为该地区大直径超长桩的理论研究和工程应用提供了宝贵的参考数据。实测结果研究表明：试桩的Q-s曲线呈缓变型,桩端承载力分担总荷载比例均低于15%,表现为摩擦桩特性;随桩顶荷载增加,桩土相对位移沿桩身的递增幅度呈先增大后减小的趋势,淤泥质粉质黏土和淤泥达到极限侧摩阻力所需的桩土相对位移分别为17和6 mm,砂土达到极限侧摩阻力所需桩土相对位移22～27 mm,桩身上部土层侧摩阻力发生不同程度的软化;桩身上部粉质黏土的桩土相对位移为18～23 mm,在桩土相对位移达40 mm时,下部粉质黏土层侧摩阻力达到极限值的87%以上,桩土相对位移继续增大时,侧阻增加趋势较为平缓,并逐渐接近于极限值;风化砂岩侧摩阻力随桩土相对位移的增加而增大,极限荷载下侧摩阻力未完全发挥;桩端阻力随着桩端沉降量的增加呈加工硬化型。     

自平衡测桩技术及其在某大厦桩基工程中的应用

针对传统测桩方法的不足之处,本文介绍了自平衡测桩方法的基本原理。并结合工程实例,详细介绍了测试流程和承载力计算方法。最后对该法的荷载传递机理及平衡点位置的选取作了简要的分析。为自平衡测桩技术在以后的桩基承载力测试中的推广应用提供了资料。     

桩顶设置弹性支座的端承桩复合桩基的设计及应用

为了实现端承桩与承台下地基土共同作用,在桩顶设置弹性支座,由弹性支座的变形实现桩基础的"刺入",使得承台下地基土能够承担上部荷载,形成复合桩基,达到充分利用承台下地基土的承载力、实现桩土共同作用的目的。本文介绍了基于此概念的复合桩基的设计思路,给出了弹性支座的设计方法,对该复合桩基承载力的安全度和变形验算等问题进行了分析和探讨,并结合工程实例,提出了桩顶设置弹性支座的端承桩复合桩基的设计建议,供工程设计人员参考。     

密实砂土层中H型锤击桩的试验研究

进行了2根H型钢桩的现场原型试验,桩长分别为39．6m和55．4m,单桩设计承栽力为3540kN。沿桩身布设了应变计后,桩通过锤击方式打入到密实砂土层中。观测了桩在不同荷载水平下的荷载传递及沉降特性。并用土力学原理对实测桩侧摩阻力的大小和分布进行了检验。     

Influence of vertical loads on lateral response of pile foundations in sands and clays

Although the load applied to pile foundations is usually a combination of vertical and lateral components, there have been few investigations on the behavior of piles subjected to combined loadings. Those few studies led to inconsistent results with regard to the effects of vertical loads on the lateral response of piles. A series of three-dimensional (3D) finite differences analyses is conducted to evaluate the influence of vertical loads on the lateral performance of pile foundations. Three idealized sandy and clayey soil profiles are considered: a homogeneous soil layer, a layer with modulus proportional to depth, and two-layered strata. The pile material is modeled as linearly elastic, while the soil is idealized using the Mohr–Coulomb constitutive model with a non-associated flow rule. In order to confirm the findings of this study, soils in some cases are further modeled using more sophisticated models (i.e. CYsoil model for sandy soils and modified Cam-Clay (MCC) model for clayey soils). Numerical results showed that the lateral resistance of the piles does not appear to vary considerably with the vertical load in sandy soil especially at the loosest state. However, the presence of a vertical load on a pile embedded in homogeneous or inhomogeneous clay is detrimental to its lateral capacity, and it is unconservative to design piles in clays assuming that there is no interaction between vertical and lateral loads. Moreover, the current results indicate that the effect of vertical loads on the lateral response of piles embedded in two-layered strata depends on the characteristics of soil not only surrounding the piles but also located beneath their tips.     

Load transfer mechanism in pile group due to single tunnel advancement in stiff clay

Construction of tunnels in urban cities may induce excessive settlement and tilting of nearby existing pile foundations. Various studies reported in the literature have investigated the tunnel–soil–pile interaction by means of field monitoring, centrifuge and numerical modelling. However, the load transfer mechanism between piles in a group, the induced settlement and the tilting of a pile group due to tunnel advancement has not been investigated systematically and is not well understood. This study conducts three-dimensional, coupled-consolidation finite element analyses to investigate tunnelling effects on an existing 2 × 2 pile group. The construction of a 6 m diameter (D) tunnel in saturated stiff clay is simulated. Responses of the pile group located at a clear distance of 2.1 m (0.35D) from a tunnel constructed at three different cover-to-diameter-of-tunnel ratios (C/D) of 1.5, 2.5 and 3.5 are investigated. The computed results are compared to published data based on field monitoring. It is found that the most critical stage for settlement, tilting and induced bending moment of pile group due to tunnelling is when the tunnel face is close to the pile group rather than at the end of tunnel excavation. The depth of the tunnel relative to the pile group has a vital influence on the settlement, tilting of pile group and the load transfer mechanism between piles in pile group induced by tunnel excavation. Tunnelling near the mid-depth of the pile group (i.e. C/D = 1.5) induces the largest bending moment in the piles, but the settlement and tilting of the pile group are relatively small. Based on a settlement criterion, apparent loss of capacity of the pile group is 14% and 23% for tunnels constructed at depths of C/D = 1.5 and at both C/D = 2.5 and 3.5, respectively. The largest load redistribution between the front and rear piles in the group and the largest tilting of the pile cap towards the tunnel occurs when tunnel excavated at C/D = 2.5.     

Numerical investigation of dynamic behavior of composite foundation composed of soilbags and piles by 3D elastoplastic FEM

In this paper, a new composite foundation composed of piles and soilbags is proposed, and its mechanical behavior is investigated systematically with numerical analyses based on sophisticated models for both soils and structures. The foundation is characterized by the laying of soilbags between the pile heads and the footing on which the superstructure stands. The expected effects of the foundation are to cut off the fixed junction between the piles and the footing in order to reduce the bending moment of the piles and decrease the response acceleration of the structure. In this study, the seismic response of the proposed composite foundation is investigated numerically by a 3D elastoplastic FEM analysis, and the superiority of the proposed composite foundation over the conventional pile foundation is discussed in detail. From the analysis, it is found that both the bending moment and the response acceleration of the pier are suppressed because of the elongation of the natural period of the foundation and the increase in the hysteresis damping of the soilbags. In addition to the above-mentioned effect on the response acceleration, the bending moment of the piles is greatly reduced at the pile heads in the proposed composite foundation due to the effect of the stress dispersion of the soilbags.     

Group effect on ultimate lateral resistance of piles against uniform ground movement

In earthquake engineering, pile foundations are designed to withstand the lateral loading that results from large displacements due to ground movement caused by strong earthquakes. The distress and failure of superstructures occurs when the lateral load exceeds the ultimate lateral resistance of the piles. The aim of this study is to estimate the ultimate lateral resistance of piles especially in terms of the group effect induced by the pile arrangement. Several experimental and numerical analyses have been conducted on pile groups to investigate the group effect when the groups are subjected to uniform large horizontal ground movement. However, the ultimate lateral resistance of the pile groups in these studies was calculated by applying load to the piles. The present study directly assesses the ultimate lateral resistance of pile groups against ground movement by systematically varying the direction of the ground movement. Although the load bearing ratio of each pile in a pile group, defined as the ratio of the ultimate lateral resistance of each pile in a pile group to that of a single pile, is an important design criterion, it was difficult to assess in past works. This study focuses on the load bearing ratio of each pile against ground movement in various directions. The use of the finite element method (FEM) provides options for simulating the pile-soil system with complex pile arrangements by taking the complicated geometry of the problem into account. The ultimate lateral resistance is examined here for pile groups consisting of a 2?×?2 arrangement of four piles, as well as two piles, three piles, four piles, and an infinite number of piles arranged in a row through case studies in which the pile spacing is changed by applying the two-dimensional rigid plastic finite element method (RPFEM). The RPFEM was extended in this work to calculate not only the total ultimate lateral resistance of pile groups, but also the load bearing ratio of the piles in the group. The obtained results indicate that the load bearing ratio generally increases with an increase in pile spacing and converges to almost unity at a pile spacing ratio of 3.0 with respect to the pile diameter. Moreover, the group effect was further investigated by considering the failure mode of the ground around the piles.     

循环水平荷载作用下的群桩性状分析

在实际工程中,很多桩基础的破坏是风、波浪或者地震等循环水平荷载的作用所致,且这些桩基础往往布置成群桩的形式。结合前人的试验模型和理论研究成果,探讨了循环水平荷载作用下桩—土之间的相互作用、循环水平荷载对桩身刚度的影响,以及桩与桩之间的相互作用效应,循环水平荷载对群桩荷载分配及群桩基础轴向承载力的影响。     

Load–settlement modeling of axially loaded steel driven piles using CPT-based recurrent neural networks

The design of pile foundations requires good estimations of the pile load-carrying capacity and the settlement. Designs for bearing capacity and settlement have been traditionally carried out separately. However, soil resistance and settlement are influenced by each other, and thus, the design of pile foundations should consider the bearing capacity and the settlement together. This requires that the full load–settlement response of the piles be accurately predicted. However, it is well known that the actual load–settlement response of pile foundations can only be obtained through load tests carried out in-situ, which are expensive and time-consuming. In this technical note, recurrent neural networks (RNNs) were used to develop a prediction model that can resemble the load–settlement response of steel driven piles subjected to axial loading. The developed RNN model was calibrated and validated using several in-situ full-scale pile load tests, as well as cone penetration test (CPT) data. The results indicate that the developed RNN model has the ability to reliably predict the load–settlement response of axially loaded steel driven piles, and thus, can be used by geotechnical engineers for routine design practice.     

砂卵石地层桩锚支护结构现场实测与研究

对成都市某楼的深基坑桩锚支护结构进行现场监测,研究分析了砂卵石地层条件下深基坑开挖过程中土压力、桩身内力、锚杆拉力的分布和变化规律。研究表明:桩侧静止土压力约占理论静止土压力的1/6～1/10,开挖过程对冠梁与底板附近的土压力影响较小;桩体钢筋应力仅发挥了钢筋设计强度的16.8%;预应力锚杆只出现了前期预应力损失等,为今后成都地区的桩锚支护结构的研究提供了较好的参考价值。